JPH0331021B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an AM stereo receiver, and particularly to an AM stereo receiver capable of receiving AM stereo broadcasts using a plurality of stereo systems in a single set.

As is well known, there are currently five AM stereo systems proposed.
Amplitude modulates (AM) the carrier wave, and phase modulates (PM) the carrier wave with the difference signal (L-R).
AM-PM method: Amplitude modulates the carrier wave with the sum signal and frequency modulates (FM) the carrier wave with the difference signal.
In the AM-FM system, two carrier waves with the same frequency and a phase difference of 90 degrees are balanced modulated using the left channel signal L and right channel signal R, and summed into a phase modulated signal obtained by adding (orthogonal modulation). The C-QUAM method applies amplitude modulation to the signal, which is also a quadrature modulation method, but uses the VCPM method to control the phase angle difference according to the magnitude of the difference signal, and converts the carrier wave into a sum signal via a 90° phase shift circuit. There is an ISB method in which amplitude modulation is performed using a signal and phase modulation is applied using a difference signal.

Therefore, the present invention provides a multi-system AM stereo receiver that can receive AM stereo broadcasts using these multiple stereo systems in a single set.

Although the above-mentioned stereo systems are based on completely different concepts even in theory, when they are examined in detail, it is found that the stereo systems have the following very common features.

(a) The envelope of the carrier wave is a sum signal with no distortion (L+
R), it follows that the sum signal can use the same envelope detector.

(b) The spread of the output sideband is compatible with the monaural case, so the phase shift is all 1.
It is less than radian (mid range).

(c) As a result of (b), subchannels of all systems, that is, difference signals (LR), can be demodulated by orthogonal synchronous detection.

After carefully considering these common points, we looked at each system from a different perspective than before, and in particular, by observing the distinctive features of the decoder configuration, we found that the five stereo system decoders can be constructed as shown in Figure 1. It can be generalized.

That is, in FIG. 1, in any stereo system, the intermediate frequency signal supplied to the input terminal 1 can be demodulated by the envelope detector 2 to obtain a sum signal (L+R), while the intermediate frequency signal A difference signal (LR) can be obtained by processing the signal in a signal processing circuit 3 according to each stereo system and demodulating it in an orthogonal synchronous detector 4. At this time, the signal processing circuit 3 has different signal processing operations depending on each stereo system, for example, AM
- To achieve compatibility with AM component removal (×1/1+L+R) in the PM and AM-FM methods, inverse modulation (×1/1+0.5 (L+R)) in the ISB method, and monaural broadcasting in the C-QUAM method. It is sufficient to perform signal processing to remove the distortion correction signal cosφ applied to the orthogonal modulated wave (×1/cosφ) on the transmitting side, and to perform variable gain (×A) in the VCPM system.

In this way, each stereo system seen from the decoder side can be characterized by the difference in the nonlinear parameter applied to the subchannel signal.
The remaining differences are simply level and phase relationships.
What is more noteworthy is that these parameters are
Except for the VCPM method, which is in the form of multiplication (or may be fixed), all the methods are in the form of division.

In view of this, the present invention makes it possible to receive all currently proposed stereo systems with substantially the same set, while switching the signal processing unique to each stereo system as necessary. It is.

Hereinafter, one embodiment of the present invention will be described in detail based on FIG. 2.

In FIG. 2, 11 is an input terminal to which an intermediate frequency signal from an intermediate frequency stage is supplied, although not shown;
13 is an amplitude limiter that makes the supplied intermediate frequency signal a constant amplitude, and 13 is a balanced mixer that outputs the intermediate frequency signal from the input terminal 11 and a signal that makes this intermediate frequency signal constant amplitude by the amplitude limiter 12. By multiplying by the mixer 13, a sum signal (L+R) is obtained on the output side. In other words, the amplitude limiter 12
and mixer 13 constitute an envelope detector.

14 is a so-called PLL circuit, which includes a phase comparator 15, a low frequency generator 16, and a voltage controlled oscillator 17.
The output from the amplitude limiter 12 and the oscillator 1
7 is phase-compared by a phase comparator 15, and the comparison error is converted into a DC voltage by a low frequency converter 16 and then supplied to an oscillator 17. The output of the oscillator 17 (oscillation frequency ) to obtain an unmodulated carrier wave sinω _c t, which is an orthogonal component. The low frequency filter 16 has time constant circuits 16a and 16b made up of, for example, a capacitor and a resistor, and these are switched by a switch 18 according to the stereo system to change the filter characteristics. In other words, the stereo system
In the case of each method other than the AM-FM method, the switch 18 is connected to the contact a side, and in the case of the AM-FM method, it is switched to the contact b side. In other words, A.M.
- In the case of the FM method, the difference signal (L-R) is pre-emphasized for τ = 400 μs on the transmitting side, so if a PLL circuit is used as the demodulation circuit,
A time delay occurs due to the time constant of the built-in low frequency filter, which may cause a phase difference between the sum signal and the difference signal, resulting in poor separation. Therefore, in the case of the AM-FM system, the cutoff frequency of the low frequency converter 16 is made sufficiently low by switching to the time constant circuit 16b side, thereby making the damping factor (usually about 0.7) of the PLL circuit 14 sufficiently low. It is made large, for example 10 or more, to approximate the first-order loop characteristic, and the pole on the high frequency side is made to be equal to the pre-emphasis time constant. Therefore, a de-emphasis circuit is not required. In addition, the time constant circuit 16a used in systems other than the AM-FM system has a time constant set so that the band of the PLL circuit 14 is narrow, for example, about 70 Hz.

19 is a divider that divides the intermediate frequency signal by a predetermined division coefficient;
The sum signal (L+R) obtained at the output side of is used, and this sum signal is selectively supplied to the divider 19 via the switch 20, taking various forms depending on each stereo system. That is, when the stereo system is an AM-PM system or an AM-FM system, the switch 20 is connected to the contact a side, and the AM cancel system using envelope division is used, thereby completely eliminating noise bursts during modulation. In addition, even when the signal is weak or exposed to pulse noise, it is possible to obtain a reception condition that is extremely similar to that of a conventional monaural receiver.Also, even when the input level drops, it is possible to completely separate the signals like a conventional decoder. There's no way Siyeon will disappear.
Furthermore, when the stereo system is the ISB system, the switch 20 is switched to the contact b side, whereby the sum signal (L+R) is attenuated by the attenuation amount determined by the resistors 21 and 22 (for example, set to 0.5). and is supplied to a divider 19, which functions as a division type inverse modulator. Further, when the stereo system is the C-QUAM system or the VCPM system, the switch 20 is switched to the contact c side and is in an open state. In other words, in the case of the C-QUAM method, division of cosφ is formally performed, but this division is not performed here. Also, regarding the VCPM method, no division is performed here. Note that 23 is a DC power supply for applying a DC bias (+1) to the divider 19.

24 is orthogonal to the output of the divider 19.
Multiply the output of the PLL circuit 14 to generate a difference signal (L-R)
This is a balanced mixer that obtains PLL circuit 1.
4 and the mixer 24 constitute a so-called PLL synchronous detector.

Further, on the output sides of mixers 13 and 24,
Phase shifting networks 25 and 26 are provided for use in the case of the ISB method. In the case of other stereo systems, it is necessary to switch these circuit networks 25 and 26, so interlocking switches 27 and 28 are provided, and in the case of systems other than the ISB system, switches 27 and 28 are connected to the contact a side. However, in the case of the ISB method, the switch is made to the contact b side. A matrix circuit 29 matrixes the sum signal (L+R) and the difference signal (L-R) and outputs a left channel signal L and a right channel signal R to output terminals 30 and 31, respectively.

Reference numeral 32 is a changeover switch for fixing the VCPM method at a predetermined angle, for example, 70 degrees.When using a method other than the VCPM method, the switch 32 is connected to the contact a side and the difference signal (L-R) is connected to the resistor. 33 to the matrix circuit 29 side (in this case, the phase angle difference of the difference signal with respect to the sum signal is 90 degrees), but when using the VPCM method, switch 32 is switched to the contact b side and the difference signal is directly sent to the matrix circuit 29 side. (In this case, the phase angle of the difference signal with respect to the sum signal is 70°).
Therefore, the value of the resistor 33 is preset so that the gain of the difference signal increases when the switch 32 is switched to the contact b side, and the above-mentioned phase angle difference becomes 70 degrees.
Note that this gain adjustment is performed using contact a of the switch 32.
The resistor 33 on the side may be removed, and instead an amplifier may be inserted on the contact b side whose gain is 1.43 (1/0.7) times that of the state on the contact a side.

Next, the operation of this circuit will be explained according to each stereo system.

First, when receiving AM-PM stereo broadcasting, switch 18, 20, 27 and 28
Connect all of them to the contact a side. In the case of the AM-PM method, the intermediate frequency signal supplied to the input terminal 11 is
It is expressed by the following formula.

(1+L+R)cos{ω _c t+(L-R)}...(1) Therefore, the intermediate frequency signal expressed by this equation (1) is directly supplied to one input side of the mixer 13, and the amplitude limiter Signal of constant amplitude through 12
The _sum signal (L
+R) is obtained. Further, when the intermediate frequency signal expressed by the above equation (1) is supplied to the divider 19, this signal is divided by the (1+L+R) signal and supplied to one input side of the mixer 24. mixer 2
A signal of sinω _c t, which is an orthogonal component obtained by the PLL circuit 14, is supplied to the other input side of the PLL circuit 14, and a signal expressed by the following equation is obtained at its output side.

cos {ω _c t+(L-R)}・sinω _c t = {sin2ω _c t+sin(L-R)} ≒sin(L-R)...(2) In the above equation (2), if LR is small, , sin(L
-R)≒LR, so the above equation (2) becomes (LR)...(3).

The sum signal (L+R) is then supplied to the matrix circuit 29 through the contact a side of the switch 27.
On the other hand, the difference signal (LR) is connected to resistor 33 and switch 3.
The signal is supplied to the matrix circuit 29 through the contact a side of the switch 2 and the contact a side of the switch 28, and the left channel signal L is supplied to the output terminals 30 and 31.
and right channel signal R are obtained.

Next, when receiving C-QUAM stereo broadcasting, first switch 18, 27, 28 and 3.
While the switch 20 remains connected to the contact a side, switch 20 is switched to the contact c side. In the case of the C-QUAM system, the intermediate frequency signal supplied to the input terminal 11 is expressed by the following equation.

(1+L+R)cos(ω _c t+φ) …(4) However, in the above equation (4), φ=tan ^-1 L−R/1+L+
It is R.

Therefore, the signal expressed by the above equation (4) is sent to the mixer 13.
When the signal cos (ω _c t + φ) which is directly supplied to one input side of the mixer 13 and obtained through the amplitude limiter 12 is supplied to the other input side of the mixer 13, it is subjected to envelope detection and a summation signal is sent to the output side. Signal (L+
R) is obtained.

On the other hand, the signal expressed by the above equation (4) is supplied as is to one input side of the mixer 24 without being divided in any way by the divider 19, and the quadrature component from the PLL circuit 14 is input to the other input side of the mixer 24. A signal of sinω _c t is supplied, synchronous detection is performed, and a signal expressed by the following equation is obtained on the output side.

In the above equation (5), cosφ is the difference signal (L
−R).

In other words, in the case of this method, formally, the above-mentioned distortion correction signal cosφ becomes a cause of distortion during demodulation, so
cosφ is detected, and the signal expressed by equation (4) above is divided by this cosφ to return to the original orthogonal modulation wave and then stereo demodulated. However, since nothing is divided here, this cosφ is used as distortion. It is included in the difference signal (LR) and output. However, this cosφ does not pose much of a problem in practice, and therefore, in this embodiment, division is not performed.

The sum signal (L+R) thus obtained and the signal expressed by the above equation (5) are supplied to the matrix circuit 29 in the same manner as in the above AM-PM system.
After being matrixed, the output terminals 30 and 3
1 as a left channel signal L and a right channel signal R, respectively.

Next, when receiving VCPM stereo broadcasting, first connect switches 18, 27, and 28 to contact a.
switch 32 to the contact b side while keeping the switch 20 connected to the contact c side. In this case, the intermediate frequency signal supplied to the input terminal 11 is expressed by the following equation.

(1+L+R) cosω _c t+(L-R/G) sinω _c t …(6) However, G is the gain coefficient and indicates the control range of the phase angle difference.
When the angle is 90° to 30°, 3.7>G≧1. The signal expressed by the above equation (6) is directly supplied to one input side of the mixer 13 as described above, and the amplitude limiter 12
is supplied to the other input side of mixer 13 through envelope detection, and a signal expressed by the following equation is obtained at its output side.

On the other hand, if the signal expressed by equation (6) above is
The signal of the orthogonal component sinω _c t from the PLL circuit 14 is supplied to the other input side of the mixer 22 and is synchronously detected. At its output side, a signal expressed by the following equation is obtained.

(1+L+R) cosω _c t・sinω _c t+(L-R/G)
sin ² ω _c t=(L-R/G) (sin2ω _c t+cos0)≒L-
R/G
...(8) Note that the gain coefficient G in equation (8) above is approximately
1.43, and by switching the switch 32 to the contact b side, the gain of the difference signal (L-R) is increased by this amount compared to the sum signal (L+R), that is, the difference signal is 70 degrees with respect to the sum signal. The signal is supplied to the matrix circuit 29 with the phase angle difference fixed at . The signal is then matrixed with the sum signal in the matrix circuit 29, and a left channel signal L and a right channel signal R are obtained at output terminals 30 and 32, respectively.

Next, when receiving AM-FM stereo broadcasting, switch 18 to contact b side and switches 20 and 32 to contact a side,
The switches 27 and 28 remain connected to the contact a side. In the case of the AM-FM method, the wave characteristics of the low frequency converter 16 are switched to the time constant circuit 16a side, so the loop gain of the PLL circuit 14 causes the voltage controlled oscillator 17 to generate intermediate frequency signals at frequencies below the time constant. However, at frequencies higher than the time constant τ, it does not follow the intermediate frequency signal, so
The signal obtained by orthogonal synchronous detection of the output of the divider 19 and the output of the PLL circuit 14 by the mixer 24 is a difference signal (L
-R) and a signal with de-emphasis characteristics corresponding to the pre-emphasis characteristics on the transmitting side, that is, the signal obtained at the output side of the mixer 24 becomes a de-emphasis difference signal (L-R).

Therefore, by supplying this difference signal (L-R) and the sum signal (L+R) obtained by envelope detection to the matrix circuit 29, the left channel signal L and the right channel signal R are output to the output terminals 30 and 31, respectively. is taken out.

In addition, regarding cancellation of AM portion, AM−
This is done in exactly the same way as the PM method.

Next, when receiving an ISB stereo broadcast, the switch 18 is switched to the contact a side while the switch 32 is kept connected to the contact a side, and the switches 20, 27, and 28 are also switched to the contact b side. At this time, the intermediate frequency signal supplied to the input terminal 11 is the sum signal (L+R)∠ +45°, which is now phase-shifted by _-45 ° on the transmitting side, _and the difference signal whose phase is shifted by +45°. If (L-R)∠ ₊₄₅ ° is Y ₊ , then
It is expressed by the following formula.

(1+X _- ) cos {ω _c t+Y ₊ (1-kX _- )}...(9) Note that in the above equation (9), k is the sub-modulation coefficient. It is said that this k is preferably about 0.5. Therefore, in this embodiment, as described above, on the receiving side, this k is set to about 0.5 by the resistors 21 and 22. Therefore, the division coefficient used for inverse modulation in this case is 1+0.5(L+R)
It is said that

Now, the intermediate frequency signal expressed by the above equation (9) is directly supplied to one input side of the mixer 13, and the signal cos{ω _c t+Y ₊ obtained through the amplitude limiter 12
₍ 1 _{- 0.5} .

On the other hand, the intermediate frequency signal expressed by the above equation (9) is divided by the division coefficient 1 + 0.5X _- by the divider 19 and then supplied to one input side of the mixer 24.
The orthogonal component sinω _c t supplied to the other input side of 4
The signal is subjected to orthogonal synchronous detection, and a signal expressed by the following equation is obtained on the output side.

(1+X _- ) cos {ω _c t+Y ₊ (1-0.5X _- )}・sinω _c
t/1+0.5X _- = (1+X _- )・sin {Y ₊ (1−0.5X _- )}
/1+0.5X _- …(10) In equation (10) above, if Y ₊ (1-0.5X _- ) is small, sin{Y ₊ (1-0.5X _- )}≒Y ₊ (1-0.5X _- ), so the above formula (10) is (1+X _- )・Y ₊ (1-0.5X _- )/1+0.5X _- = 1+0.5X
_- -0.5X ² _- /1+0.5X _-・Y ₊ = (1-0.5X ² / _- /1+0.5X
_- )・Y ₊ …(11). Here, if 0.5X ² _- ≪1, X _- <0.5,
The above equation (11) becomes ≒Y ₊ = (LR) ∠ ₊₄₅ ° (12). In this case, the distortion included in the difference signal Y ₊ is 0.5X ² / _- /1 + 0.5X _- ·Y ₊ from the above equation (11).

The sum signal (L + R) ∠ _-45 ° obtained at the output side of the mixer 13 and the difference signal (L - R) ∠ ₊₄₅ ° obtained at the output side of the mixer 24 are respectively transferred to the phase shift circuit 25.
and 26 to a matrix circuit 29, where it is matrixed, and a left channel signal L and a right channel signal R are taken out at output terminals 30 and 31, respectively.

Note that in the process of obtaining the difference signal (LR) in the ISB method, an envelope divider is used as in the AM-PM method, and the division level is 1/2 that of the AM-PM method. Therefore, the AM portion remains, but this
The AM component is the distortion function (1−kX _- ) multiplied on the transmitting side.
This effectively cancels the

As described above, according to the present invention, it is possible to receive all five currently proposed stereo systems with a single set by simply switching the necessary switches according to the stereo system, making the configuration simple and inexpensive. You can get a versatile AM stereo receiver.

Each switch may be switched manually, automatically switched by detecting a pilot signal, or stored in the station memory and switched based on this information.

[Brief explanation of the drawing]

FIG. 1 is a diagram for explaining the basic principle of the invention, and FIG. 2 is a circuit configuration diagram showing an embodiment of the invention. 12 is an amplitude limiter, 13 and 24 are balanced mixers, 14 is a PLL circuit, 18, 20, 27, 2
8, 32 are switches, 19 is a divider, 25, 26
is a phase shift circuit network, and 29 is a matrix circuit.

Claims (1)

[Claims] 1 Envelope detection means for obtaining a sum signal (L+R) from an intermediate frequency signal and a signal with a constant amplitude of the intermediate frequency signal, and a signal obtained by passing the constant amplitude signal through a PLL circuit and the intermediate frequency signal according to a stereo system. a synchronous detection means that obtains a difference signal (L-R) from the signal divided by a division coefficient that is switched by a division coefficient, and a matrix circuit that mixes the sum signal and the difference signal to obtain a left channel signal L and a right channel signal R. , and a phase shift circuit network inserted between the matrix circuit and the synchronous detection means only in the case of a specific stereo system.